Just to elaborate a little on what Draco answered :

http://i609.photobucket.com/al [...] tumntornado/Clickherething.png

This is basically a screecap of what the forum looks like when you're posting a topic (or comment). I circled the places Draco said to click, that's pretty much all I did.

I figured a visual may help answer your question =)

If you mean on the forums, you can do one, what Mighty posted, or:

below the text box there's an 'upload attachment' area for browsing your computer for the file, if its on your hard drive.

After you find the file, click 'add file', position your cursor where you want it posted, and click the 'place inline' just below the text box. You can place several images this way, just be sure to reselect the different picture from the drop down menu.

Use the code [img]inserturlhere[/img]